In the field of mid infrared spectroscopy of solids a significant problem which has been encountered is the preparation of samples of the solids which are sufficiently thin to transmit significant amounts of infrared radiation in order to enable the spectroscopic analysis to take place. A known way of attempting to deal with the problem has been to abrade the solid material using a commercially available abrasive paper, e.g. sandpaper, which typically incorporates particles of SiO and Al.sub.s O.sub.3. This creates on or in the sandpaper small particles of solid material to be investigated. The sandpaper is then placed in an instrument which can be used to carry out the spectroscopic analysis and a spectrum of the sandpaper is measured in diffused reflectance in a known way. This reveals the characteristic spectrum of the sample, in particular the small particles of the sample embedded in the sandpaper.
An enhancement of this method is to use sandpaper coated with a relatively thin coating of high reflective material such as aluminium.
It has been the practice to use small discs of sandpaper for forming samples as described above. Such small discs are satisfactory when the surface of the material to be analysed has projections such as a corner against which the abrasive surface can be rubbed to form the sample on the surface of the sandpaper. However, for very flat surfaces or for surfaces where there are recesses, it can be more difficult to obtain a satisfactory sample using the sandpaper discs. The present invention has been devised in order to deal with this problem.